Methods for manufacturing fibrous structures



Oct. 31, 1961 J. H. WAGGONER METHODS FOR MANUFACTURING FIBROUS STRUCTURES 2 Sheets-Sheet 1 Original Filed Sept. 12, 1950 INVENTOR. Jack H Plzggarzer BY Oct. 31, 1961 J. H. WAGGONER METHODS FOR MANUFACTURING FIBROUS STRUCTURES 2 Sheets-Sheet 2 Original Filed Sept. 12, 1950 Jack fl: W EP az'ilys United States Patent ice 3,006,805 lVfli/IHODS FOR MANUFACTURING FIBROUS STRUCTURES Jack H. Waggoner, Newark, Ohio, assignor to Owens- Corning Fiherglas Corporation, a corporation of Delaware Original application Sept. 12, 1950, Ser. No. 184,355, now Patent No. 2,772,603, dated Dec. 4, 1956. Divided and this application Aug. 17, 1956, Ser. No. 609,338

Claims. (Cl. 162-401) The present application is a division of my co-pending application Serial No. 184,335, filed September 12, 1950, now Patent No. 2,772,603.

This invention relates to glass fiber products and particularly to the manufacture of thermal insulation, sound insulation, structural board, and fibrous sheets which may be used as such or in the manufacture of laminates, mats and bats and the like, and in which glass fibers constitute the major constituent. This invention also contemplates the use of other fibers which have some of the characteristics of glass fibers, such, for example, as fibers drawn from synthetic resins.

The use of attenuated fiber of glass or synthetic resin in the manufacture of fibrous structures is faced by the difiiculty of integrating or otherwise forming the glass fibers into a self-sufficient mass. This is because the glass fibers and the like exist in the form of rod-like filaments of substantial length having little, if any, crimp or curl and having perfectly smooth rounded surfaces. There is nothing on the glass fiber surfaces which might cause the fibers to cling together upon contact with one another, and, therefore, it is difficult to felt the fibers into a self-suflicient mass. Because of the smooth, nonporous nature of the glass fiber surfaces coupled with their hydrophobic characteristics, it is difiicult to bond resinous materials or other adhesive to the glass fiber surfaces for the purpose of integrating the fibers one with another. Although these properties may be used to advantage in some applications, they detract from or impair the assembly of the fibers into fibrous structures having a high degree of mass integrity.

By way of comparison, other common natural fibers, such as wool, cotton and the cellulose fibers, have their surfaces covered with a large number of tiny fingers or hairy projections which apparently cause the fibers to cling to one another upon contact. This agglomerating or combining characteristic permits felting together in a dry state or from liquid suspension to form fibrous structures of considerable strength. In fibrous structures of this type, it is often unnecessary to make use of additional binder in the form of resinous materials or the like to achieve strength sufiicient for the purpose for which the structure is intended.

0n the other hand, the inability of glass fibers and the like to interfelt into a mass with sufiicient strength and integrity to resist forces to which it might be exposed as an incidence to normal handling, makes it necessary to incorporate binders or cements to gain a limited degree of bond between the fibers, whether in the form of a mass, web, yarn, strand or other preformed body.

Aside from the usual high cost of the adhesive or binder and its application, it has often been found necessary to employ such large amounts of binder in order to effect a desired degree of mass integrity that other desirable properties of the structure are handicapped. For example, most organic materials which might be used as binder compositions begin to decompose by thermal reaction upon exposure to temperatures of 400 to 500 F. so that products in which they are embodied are limited to low temperature applications. This is important because, in the absence of such organic binder, heat insula- Patented Oct. 31, 1961 tion formed of glass fibers could be subjected to temperatures in excess of 1000" F. Without harm to the fibers or the structure. More important by Way of further illustration, high concentrations of resinous material, such as phenolformaldehyde, so stifiens and embrittles the glass fiber mass that it detracts from its acceptability as an insulation or textile product. It reduces its recovery and also greatly reduces its tear and strength properties.

Attempts have been made to make use of inorganic binder compositions, such as bentonite and other clays, for the purpose of maintaining the inorganic character of the final product while gaining the benefit of the mass integrity imparted by the use of such binder compositions. While binding materials such as these are resistant to high temperatures, they exhibit high attraction for moisture and are hygroscopic, and since it is the usual practice to apply such compositions in aqueous medium, removal of the diluent and the moisture combined in the binder material requires considerable heat treatment. In clay systems, it is necessary to use as much as 15 percent by weight bentonite or the like; Such amounts cause gelation in the fibrous structure upon formation with aqueous medium and it is practically impossible to achieve proper ventilation for the removal of water at a rapid rate during the final steps in manufacture. Reliance must be had upon surface evaporation, which is slow and expensive. Furthermore, the presence of such large amounts of clay results in a product which is dusty, brittle and friable and therefore limited in application as well as fiexure strength and tear strength.

It is an object of this invention to produce a fibrous structure of attenuated glass or synthetic resinous fibers and the like having high mass integrity with little, if any, adhesive, and it is a related object to provide methods for producing same.

Another object is to produce glass fiber structures of glass fiber modified in its surface characteristics to cause the fiber to cling together in the formation of a felted mass having high integrity and which may be markedly increased in strength thereafter by the incorporation of a minimum amount of resinous binder, and it is a related object to provide a method for producing same.

A further object is to produce glass fiber products in which the fibers may be formed into a se'lf-sufiicient mass of high strength by the use of a bonding agent that does not detract from the use of the finished article for purposes of the type described.

A still further object is to produce a fibrous product of the type described employing a small amount of a binding agent which is fibrous in character, and it is a related object to produce a fibrous structure of the type described in which the binding agent operates flexibly to space the glass fibers one from another to increase flexibility and reduce the possibility of self-destruction by mutual abrason.

A basic object is to combine attenuated glass fiber with felt bonding fibers which become associated or orientated with the glass fibers to enable the arrangement thereof into a structure having high mass integrity and which protects the glass fiber surfaces against abrasion of the type which has heretofore limited the use of glass fiber in textiles, mats, or batts.

A still further object is to produce a glass fiber product of the type described in which the binding agent consists chiefly of a fibrous material which is present in such small quantities as to permit the product to retain practically all of the original characteristics of the glass fibers, such as flame-proofness, heat resistance, and resistance to exposure at high temperature.

A still further object is to produce glass fiber products of the type described which may be varied in density from 3 below three pounds per cubic foot to thirty pounds per cubic foot (or more) and which may be further treated or V a A still further object isto produce fibrous insulation and structural boards and other preformed shapes characterized by high temperature resistance, incombustibility, high tensile strength, high flexure and impact strength, and nail holding ability while having suflicient porosity to-serve as sound and thermal insulation, and it is a related object to provide new and improved processes for manufacturing same.

An object of this invention is to provide an improved binder system for glass fiber products and to provide new processes making use of same in the manufacture of fibrous structures. a r V A still further object is to produce and to provide a method for producing a bonded fibrous structure having rnicroscopicbubbles or voids arranged in substantially uniform distribution adjacent the glass fiber surfaces to achieve the properties of a highly bulkedup glass'fiber structure with minimum concentration of binding agent.

A still further object is to produce a porous glass fiber structure embodying expanded or puffed mineral as a bulking agent to increase porosity without detracting from the other properties of the fibrous structure,

A still further object is to produce and to provide a method for producing a fibrous structure loaded with microscopic bubbles and to employ ultrasonic and supersonic vibration in the manufacture of same.

7 These and other objects and advantages of this in vention will hereinafter appear and for purposes of illustration, but not of limitation, embodiments of this invention are shown in the accompanying drawings, in which- FIGURE 1 is a schematic view of a technique which may be used in the practice of this invention;

FIGURE 2 is a schematic view of apparatus for manufacturing'bubble filled fibrous products embodying featuresofthis invention;

' FIGURE 3 is a schematic sectional elevational view of another form of apparatus for carrying out this invention. 7

As used herein, the term binder fibers relates to fibers having large numbers of fuzzy ends, hairs or flexible fingers extending from the sides which cause the fibers 'to cling to each other and interlock upon contact, especially after being felted from a highly dispersed condition and then dried. Characteristics such as these are exhibited by natural fibers such as cellulosic fibers derived from wood, paper pulp, cotton, corn stalks, sisal, hemp and the like, and from inorganic fibers such as asbestos fibers of the type amosite, chrysotile and materials of the type Paligorskite, commonly known as mountain leather. Use as a binder fiber may also be made of synthetic fibrous minerals such as rock wool, synthetic as-' bestos fibers and the like, and even glass'under certain circumstances which will hereinafter be described. Although for many purposes such fibers may be usable interchangeably or in combination with each other for binding purposes in accordance with the concepts of this invention, there are situations wherein such substitution cannot be made because the process is applicable only to asbestos type fibers or the like or to cellulose pulp fibers, or the like, as will hereinafter be developed.

The term glass fibers, as used herein, is meant to include glass fibers of the staple or wool type, such as are attenuated from molten streams of glass by reaction with high pressure air or steam. Included also are continuous fibers mechanically drawn at high speed from streams of molten glass and which may cut to desired lengths for manufacture of fibrous structures embodying the concepts of this invention. It has been found that structures embodying features of this invention may be prepared of glass fibers of considerable length er of relatively short length, and that very oftenmost satisfactory results are secured by'the use of glass fiber having considerable variation in length between long to very short fibers of a few microns in length. Glass fibers of the type described may be used in the condition in which they are delivered from the fiber forming unit-that is, with or without lubricant or size thereon, but it is preferred to remove the size or coating prior to use in the formation of fibrous structures in accordance with this invention. Porous and bonded mats or batts of glass fibers may also be used to form structures having new and improved characteristics embodying features of this invention, as will hereinafter be described. a A basic concept of this invention resides in the orientation of binder fibers in small proportion with the glass fiber surfaces whereby the fibers together are able to form into a felted mass and interlock in a manner to provide sufi'icient strength to resist forces incident to normal handling and use. The desirable properties of the glass still remain in that the structure is' substantially incapable of combustion, it is able to resist temperatures in excess of 1000 F., it has high strength, is rot-proof, vermin-proof, and substantially inert, and is admirably adapted for use as sound insulation, heat insulation, resinous reinforcement, coated fabrics, textile fabrics, and. the like. The desirable properties of the small amount of binder fibers are clearly evident in that the fibers in the structure cling to eachother strongly. Y

The reasons why such small amount of binder fibers are able to impart the desired results have not yet been fully developed. It appears, however, upon examination, that the glass fiber surfaces become covered substantially throughout with the felt binding fibers and become so well integrated with the glass fiber surfaces as to appear as a part thereof and impart felt bonding characteristics thereto. a

The forces by which the binder pulp fibers integrate with the glass fiber surfaces appear to be greater than that which results from merely felting out thepulp fibers onto the glass fiber surfaces. In addition, the coverage of the glass fiber surfaces with what appears almost as a monomolecular layer of the pulp fibers substantiates the concept that the pulp fibers, such as kraft fibers, carry a negative charge (anionic), while portions of the glass fiber surfacm remain positively charged (cationic), whereby a type of ionic bond forms between the kraft and glass fibers-the like charges in the kraft pulp fibers causing the fibers to repel each other in aqueous dispersion and to be strongly attracted to the glass fiber surfaces until the charges thereon are substantially completely satisfied upon deposition of a layer of binder fibers thereon. Thus physical-chemical forces are involved in the original orientation of the colloidal pulp fibers with the glass fiber surfaces to the end that the glass fibers are substantially completely covered with a small amount, compared to the weight of the glass fibers, of binder pulp fibers. The glass fiber so modified is capable of felting in the manner of the pulp fibers disposed thereon. This is evidenced by the fact that a small percentage of colored pulp fibers, less than 5 percent by weight of the glass fibers, are able to cover the glass fibers so .complete throughout when deposited from dilute aqueous solution and dried that the glass fiber appears to be of the same color throughout and becomes invisible per se by the naked eye. For example, whenlO parts of a yellow pulp fiber was mixed with- 1000 parts of water and parts by weight of glass fiber was mixed with 20,000 parts by weight water and the two dispersions combined, the fine pulp fibers begin to bond and felt all over the glass fiber surfaces to impart to them the same color and the characteristics of the felt binder fibers. These pulp. fibers arrange themselves all around the glass fibers and thereby enable the glass fibers so coated to felt together into a fibrous structure of high mass integrity.

For best operation, it is desirable to so thinly disperse the pulp binder fibers in the aqueous medium that they are able freely to move about and float away from each other. The separation of the pulp fibers is encouraged by the ionic forces which cause them to repel each other while binding with the glass fiber surfaces is encouraged throughout by the attraction that exists between unlike charges.

It appears that upon drying, some shrinkage occurs as the pulp fibers form into a porous interfelted network about the glass fibers, whereby the pulp fibers retreat in part to the glass fiber intersections. Thus the dried pulp fibers concentrate at the glass fiber intersections where they are more ably adapted to achieve their interbonding function. It appears further that the highly dispersed network of pulp fibers lose their colloidal properties and shrink to an open but tangled mass completely interlocked with the glass fibers. At the same time, the network of glass fibers keep the fibrous structure from shrinking to a higher bulk density in the absence of compression. The pulp fibers integrated all around the glass fiber surfaces and especially at the glass fiber intersections operate not only to tie the glass fibers together but also maintain a desired spaced relation between them. In this way the abrasive effect, such as mutual abrasion, is greatly reduced to make the felted fabric particularly useful in the textile art.

Microscopic examination reveals the presence of pulp fiber nests at the points of intersection between glass fibers in the endproduct. These concentrations at glass fiber intersections may result, in part, from the reactions described above, but the amount indicates a further settling as by felting, filtration or merely settling out at points of glass fiber contact where the presence of such high concent-rations greatly benefit the characteristics of the final product. They not only provide greater bond where it will be most effective, but they function as spacers more fully to separate the fibers in the fabric and protect the fibers from destruction by mutual abrasion. This, in part, is responsible for the greater flexibility of the fibrous structure.

Though mass integrity flows from the practice of the invention as described, it has also been found that a small amount of resin may be used in addition greatly to increase the strength far beyond that which results from the use of an equivalent amount of resin in ordinary glass fiber or felted fibrous structures.

The length of the glass fiber is not controlling except for the understanding that strength generally is in proportion to length and, as previously pointed out, highest strength results from the use of glass fibers which vary in length from very short to fairly long. It is best if the pulp binder fibers are reduced to the smallest length possible. Lengths less than 5 inch may be used but the amount that is necessary is inversely proportioned to length. Similarly, equivalent strength results from the use of less pulp fiber which has been more finely pulped.

It has been found sufiicient if the amount of pulp or binder fibers present in the fibrous structure constitutes as little as 3 percent by weight, but best results are secured when used in amounts ranging from 5-15 percent pulp fibers of the asbestos type or 15-25 percent of the asbestos type. When cellulose type fibers are used in amounts set forth, the resulting fibrous structure retains the high temperature resistance and the flame-proofing characteristic of glass fibers. When asbestos constitutes the binding medium, the resulting structure is not only flame-proof but it is unaffected when exposed to temperatures as high as 1200" F., or more. Very desirable structures can be manufactured with pulp binder fibers present in amounts ranging as high as 30 percent by weight, but further amounts of pulp fiber seemingly have a dele- 6 terious effect upon the strength, heat resistance, and electrical insulation'characte'ristics of the resulting fibrous structure.

For example, 5 percent of highly pulped kra'ft fiber (about 1-10 microns) is just as efiective as a binder in the manufacture of a fibrous structure as 8 percent of such fibers which have not been pulped to as great an extent (about inch). The difficulty encountered with the use of highly pulped fibers resides in the freeness of the formfiber structure. This relates to the ability rapidly to remove the aqueous medium from the mass formed upon separation of the fibers from suspension. When the binder fibers are used in the amounts described, freeness or elimination of the water from the deposited fibrous structure does not present a problem because it is possible to suck hot air through the deposited fibrous structure so that drying can be effected in relatively short time. In this manner moisture can be eliminated and the product dried in a matter of 5-15 minutes.

Particular importance is attributed to the fact that the longer and comparatively coarser glass fibers maintain suflicient spacing in the fibrous mass to permit the major portion of the free water to drain while leaving an open structure through which air can be drawn to accelerate evaporation of the remaining water. The air can be heated further to reduce the drying time. By way of comparison, when clay binder systems are used, the highly gelatinous character of the clays causes the fibrous structure to become completely plugged so that internal ventilation is substantially impossible. 'This is true with clay contents even as low as 8 to 10 percent by Weight. As will hereinafter be described, clay may be incorporated as an ingredient in fibrous structures embodying features of this invention, primarily for the purpose of improving fiame-proofness, but in that event, the predominance of the glass and pulp fibers still provides for an open structure.

Excellent results are secured by the use of pulped newsprint as the binding medium because the lignin, the natural binder in the cellulose product, remains and is able to function as a binding agent, especially after heating. When kraft pulp fibers are used from which lignin has been removed, improved results are secured if the lignin is later replaced by incorporating with the natural cellulose binder in the aqueous slurry or by incorporating the lignin in the formed fibrous structure. It appears that the lignin incorporated in this manner concentrates at the fiber intersections where it is better able to function for the purposes for which it is intended. It will be understood that when other pulp fibers of'the type described are embodied as the binding material, lignin as well as other resinous material may be used to improve the strength properties of the fiber structure, as hereinafter will be described.

The following examples are illustrative of typical formulations embodying 'concents of the invention so far described:

Example 1 0.12 pound kraft pulp (6% fiber in aqueous medium) 4 pounds white wool glass fiber 132 pounds water This formulation results in the manufacture of a fibrous structure having about 3.3 percent pulp fiber and 96.7 percent glass fiber. The fibrous structure has excellent strength, especially in the lengthwise direction, and is able to serve as a structural board when compressed sutficiently, as will be described.

Example 2 10 parts by weight pulped kraft fiber parts by weight continuous glass fibers cut to short lengths 4000 parts by weight water Example 3 10 parts by weight asbestos pulp waste 50 parts by weight glass fiber Water in amount to make up 2% concentration of fiber in the dispersion Examples 4 10 parts by weight Paligorskite 40 parts by weight glass wool fibers Water to make up about 0.3% dispersion Example 5 parts by weight kraft pulp 4 parts by weight lignin 150 parts by weight continuous fiber cut to /z inch lengths 50,000 parts by Weight water The systems which will now be described illustrate various inventive concepts which may be used to manufacture fibrous structures with formulations of the type described. a 1

Example 6 A still further method which may be used in the practice of thisinvention is schematically illustrated in FIG- URE 1 of the drawing. In this system, the desired amount of glass wool with or without binder or size is fed continuously as a felted web 50 along a conveyor 51 into a tank 52 containing a slurry 53 of the pulp fibers in the ratio described. The slurry is kept under constant agitation in the tank so as the keep the pulp fibers in uniform distribution by recirculating the slurry from the underside of the tank through conduits 54 and 55 to readmit the slurry with any makeup admitted from reservoir 55 through an inlet 56 in the upper region of the tank. Such circulation generated by pump 57 also causes the slurry to pass through the layer of wool fibers as they are submerged by rollers 58" and carried lengthwise through the tank along an endless conveyor 59. Such forced circulation of the slurry through the web of glass fibers insures the infiltration of the pulp into the innermost regions of the fibrous layer so as to enable the pulp fibers to become fully oriented and cover the glass fiber surfaces and concentrate at the intersections for better binding purposes. In the treatmentof glass wool fibers, it is best to make use of dispersing agents such'as 'Will hereinafter 'be described to enhance penetration'and minimize filtering out the pulp fiber on the wool mass. It is still better tomake use of materials which increase the penetrating characteristics of the slurry, such, for example, as the fiocculating agents or like substances which seem to eliminate, temporarily, the fibrous nature of the pulp fibers united deflocculated, as will also be described hereinafter.

Excess slurry and water is expressed between squeeze.

rolls 60 and 61 as the fibrous mass is'carried out of the tank by the conveyor 59. The fibrous layer 62 thus formed may be advanced to compressing means for densification or else may be advanced directly to a drying oven or other processing elements; i

- It has been found that intaglio and raised designs may be permanently formed in the fiber structure by depositing the fibers or by compressing the fibrous layer during drying on a correspondinglycontoured surface. Instead, the fibrous structure may be formed between plates to have such raised or grooved designs so long as about l-3 percent free moisture remains in the final product. Other shapes may also be formed as an incidence to the manufactureof the fibrous structure or afterwards so long as the desired amount of moisture is present. For example, compressed boards may, be, formed with grooves in one end and tongues in the other which interfit toestablish a desired relation. I V

a It has also been found'that the density of the formed fibrous structure can be increased. when desired or the incombustibility and heatresistance of the fiber structure may be improved, or both characteristics may be achieved at the same time by the introduction of colloidal or finely-divided inorganic substances into the bonded fibrous structure. Such inorganic loading agents may be formulated as a constituent of the fibrous suspension for use in the formation of the fibrous layer, or they may be separately introduced later. For such purposes, use maybe made of inorganic loading agents such as phosphate clays, china clay, bentonite, colloidal metallic oxides, expanded perlite, exfoliated vermiculite, diatomaceous, earth, finely divided glass fibers, dicalite, silica and the like. I Densities in the range of 10 to 30 pounds per cubic foot may be achieved in the fibrous structure, depending upon the concentration of the loading agent and the amount'of compression of the fibrous layer during drying. ,Without a loading agent it is diflicult to achieve specific gravities higher'than 30 pounds per cubic foot. With the use of expanded minerals of the type described, reduced combustibility is achieved coupled with reduction in density of the type desired in insulation materials. Fibrous structures having as much as 10 percent organic pulp binder fibers and loaded with inorganic particles of the type described are able to Withstand extended exposure at elevated temperatures, with asbestos pulp, exposure may be at temperatures as high as 1400" F. Color of a prominent character may be embodied in the formed fibrous structure by judicial selection of metal oxides, such color systems are especially desirable because of their permanent character and the ability to gain uniform distribution of the coloring particles throughout the fibrous mass so'that depth of color and color intensity can be achieved. a

Oneof the difiiculties encountered in the manufacture of fibrous structures of the type described resides in the inability always to maintain a uniform dispersion of the pulp and glass fibers. Settling out or nonuniform distribution tends to take place, especially when cellulosic and glass fibermix-tures are used. It has been found that the stability of the dispersion may be greatly increased by the addition of finely divided loading agents, especially-the clay or earth minerals. These tend to aid the dispersion and cause better distribution of the materials in the separated fibrous layer. When the pulp fibers are to be added into already formed glass fiber layers or fibrous structures of the type described, the presence of such inorganic dispersion agents permits fuller penetration with minimum separation on the surface of the felted structure. For this purpose, from 1 to 5 percent dispersing agent is satisfactory, but more may be used since the entire quantity may also serve as a bulking or loading agent as described. As a loading or bulking agent, up to 30 or 40 percent finds beneficial use, especially with organic binder fibers to reduce their combustibility and to increase density.

Concepts of this invention may also be practiced with glass fibers which have already been bonded into a fibrous structure with other medium. In that event, the pulp binder fibers function as an additional binding agent and further improve the strength and flexibility characteristics of the product. The diificulty with the after incorporation of the pulp fibers in bonded glass fiber structures resides in the uniform distribution of the pulp fibers into the interior of the fibrous structure. The pulp fibers have a tendency to filter out onto the surface of the web or mass. In order to minimize separation of the pulp fibers on the outer walls of the glass fiber structure and in order to assist penetration thereof into the bonded or unbonded glass fiber structure, materials having the ability of reducing the'viscosity of the slurry are used. Generally, the viscosity of the slurry may be varied by giving consideration to the ionic or acidic nature of the slurry. For example, if the slurry is applied to the glass fiber structure within a pH range just below 7, the viscosity is lowered to enhance penetration. It been found that such fibers may be coagulated after penetration has been achieved by changing the pH to about 7-8.5, such as by the addition or after treatment with solutions of ethylamine or other amines, sodium phosphate or other alkaline substances of like nature. Throwing the slurry further onto the acid side by the addition of stronger acids, such as hydrochloric acid, sulfuric acid, or the like, will also coagulate the fiberin position of use.

When such fibers, particularly asbestos fibers, are incorporated in glass fiber mats bonded with organic resins, it is possible to expose the structure to elevated temperatures to burn out the resinous material and still maintain a satisfactory bond because of the binder fibers in the fiber structure.

It has been found further that in the use of an asbestos slurry, bentonites and other corresponding clay materials are able to assist the penetration of the asbestos into the glass fiber structure. Upon heating, orientation between the asbestos and bentonite seems further to improve the bonded nature of the inorganic product. By the use of this technique, bentonite may be incorporated in amounts ranging from -50 percent in the product and the product can be exposed to temperatures as high as 1200 F. without change.

The inventive concept which will hereinafter be described is applicable to fibrous structures wherein asbestos type fibers comprise the principal binding medium. It has been found that if the aqueous medium in which the asbestos fibers are dispersed is adjusted by certain acidic compounds, taken alone or in combination, the fibers have a tendency temporarily to deflocculate so that their fibrous character is minimized and the dispersion becomes more fluid. When this character has been imparted to the asbestos pulp fibers, the dispersion is capable of penetrating substantially completely into a fibrous mass without filtration of the asbestos felt fibers onto the surfaces thereof. The acidic nature of the dis persion functions at the same time to impart another beneficial effect in that it makes the glass fiber surfaces more receptive for the bonding agent and the acid reacts with the materials on the glass fiber surface to make available greater surface area to which the pulp fibers might integrate. When the slurry is adjusted to a pH which is varied slightly below neutral, the fibrous character of asbestos is falsified and the slurry becomes very fluid. The pulp fibers may be coagulated or caused to reappear in their natural form upon further decrease of pH by the use of acid or by increase of the pH as to between 7 to 8.5 by the addition of basic substances or use may be made of other known coagulating agents.

Compounds capable of defiocculation of the type described with asbestos pulp fibers may be selected of organic and inorganic compounds and mixtures thereof, such as tanning acids and their salts represented by tannic acid and its salts, citric acid and its salts and the like; metal chlorides, metal sulphates and metal salts represented by aluminum chloride, magnesium chloride, zinc chloride, ferrous sulphate, chromium nitrate, chromium chloride, chromium sulphate, stannic chloride, ferric chloride and the like; and combinations of such organic acids and their salts with the inorganic metal salts of the type described. Further improvement is achieved by the use of alkylolamines with one or more of the described organic or inorganic acidic substances. Representative of suitable alkylolamines are ethanolarnine, diethanolamine, tri-ethanolamine and the like.

The concepts described above are particularly well adapted for the incorporation of asbestos fibers into bonded glass fiber products since better penetration to secure uniform distribution of the asbestos fibers amongst the glass fibers may be achieved. Separation of the asbestos onto the glass fiber surfaces may be effected by the techniques previously described or by providing the bonded glass fiber surfaces with a dry coagulating agent or by fiocculating the asbestos slurry after full 10 penetration has been achieved. If desired, such further additions of pulp fibers may be achieved in fibrous structures which have already been formed in accordance with the concepts of this invention earlier described. In this manner the amount of pulp fibers might be increased for the purpose of improving the bonding relation.

By way of further modification, resins such as polyvinylidene chloride, phenol formaldehyde, polyvinyl chloride and the like resins which are unaffected by the acids may be incorporated with the slurry of reduced viscosity. When resinous materials of the type described are incorporated, they may be activated at elevated temperatures, such as by ironing the fibrous structure between heated rolls or upon drying and baking at temperatures in the range of 300-500 F.

Specific examples of various formulations embodying this phase of the invention are as follows:

Example 7 Grams Calcium aluminate 5 Asbestos (chrysotile) 10 Tannic acid 0.2 Water 500 Example 8 Asbestos (amosite) 5 Aluminum sulphate 1 Magnesium chloride 0.2 Tannic acid 0.0 Water 500 Example 9 Asbestos (chrysotile) l0 Ethanolamine 0.4 Tannic acid H 0.2 Water 500 Example 10 Asbestos (amphobile) 5 Aluminum chloride 1 Water 500 Example 11 Asbestos (1 micron to 10 Hydrochloric acid solution 1 Water 500 Example 12 Asbestos (chrysotile) 10 Ferric chloride 1 Chromium nitrate 0.5 Water 500 The above examples, and particularly Example 7, may be formulated by beating the materials together for about five minutes to form an excellent dispersion in which little, if any, settling takes place. When mixed with glass fibers in the desired proportion and dried in a felted layer, the asbestos pulp fibers deposit over the glass fiber surfaces and form nests .at the fiber intersections upon drying to provide an excellent bonding agent.

Applicant has found a number of novel systems which may be employed in the fabrication of bonded fiber structures based upon the principles hereinabove described.

In a preferred system, the deflocculated pulp fibers in admixture with the desired quantity of glass fibers are coagulated or precipitated by coagulating agents just in advance of the separation of the fibers in a separating wall. In this manner excellent distribution of the pulp and glass fibers is secured throughout to provide a uniformly bonded fibrous structure. Very little, if any, fiber passes through and that which does may be returned to the process by reuse of the water or by recirculation.

11 When so deposited very little, if any, fiber is thereafter removed by subsequent applications of wash water. a

In a desirable system, glass fibers may be first deposited in a layer with a coagulating chemical dried on the surfaces thereof. When the dispersion of reacted asbestos fibers has passed through the glass fiber layer and the asbestos contacts the wetted glass fiber surfaces, the asbestos fibers are coagulated immediately and attach themselves firmly to the glass fiber surfaces.

Fibrous structures of the type produced by the concepts just described, composed almost entirely of inorganic materials, are capable of withstanding temperatures as high as 1200 F. and remain unchanged under direct flame.

It "is well known that maximum separation between glass fibers is desired to achieve greatest utility of the glass fiber structure. This permits high fiexure of the fiber structure without fear of destruction of the fibers by mutual abrasion. It has been the practice to secure fiber separation of the type described by the use of large amounts of resinous material or by the use of large amounts of loading agents, alone or in combination with the resinous materials.

The use of highconcentrations and proportions of resinous materials is undesirable because of the high cost of such materials and because of the stiffness and inflexibility which such resinous materialsnaturally impart to the fibrous structure. Loading agents, on the other hand, detractfrom manyof the other properties of the glass, fiber structure when present in high concentration.

Applicant has found an entirely new system to achieve separation between fibers in the manner accomplished by the use of large amounts of resin without being handicapped by high cost or loss of desirable properties of the fibrous structure. .The desired separation may be achieved more satisfactorily by means which cause the development of a large. number of bubbles of micro scopic size, especially in the regions adjacent the glass fiber surfaces or in the bonding agent.

It is an object here to improve the quality of glass fiber structures by providing a large-number of microscopic bubbles in the fibrous structure to reduce the specific gravity and increase the bulk of the product while reducing the possibility of abrasion between fibers by increasing the distance between them. The presence of such microscopic bubbles between the fibers increases the radius of curvature through which the fiber may bend under conditions of use, and protects the fibers so that force applied at any one point is cushioned and distributed throughout the length of the fiber and to adjacent fibers.

There are a number of techniques which have been developed for accomplishing the desired relation in the fibrous structure.

(l) The development of microscopic bubbles in the binder composition can be accomplished by subjecting the binder itself to high temperature, electrical or chemical treatment in such manner as to cause the release of gases from elements therein, or a binder having microscopic bubbles already provided therein may be used for securing the fibers one to another in the mass.

(2) Microscopic bubbles may be released from'chemical compounds such as from particles of calcium carbonate or other gas forming chemical compound which may be incorporated as part of the hinder or as a loading agent with'the fibers during formation of the fibrous structure. The liberation of gas bubbles may be accomplished just before final formation of the fibrous structure, during formation or subsequent to its formation. Liberation of the miscoscopic gas bubbles in position of use may be accomplished upon exposure of the chemical compound to acidic medium, by heating to relatively high temperature in the range of 500-1200 F. for a short time, or by ultrasonic or supersonic vibration.

' For example, when the chemical compound is in'position of use, the structure may be exposed to acid forming gases such'as vapors of hydrochloric acid or the like. Instead, it may be incorporated with acid forming compounds such as the metal chlorides of the type aluminum chloride, 'iron chloride, tin chloride and the like, the acid being formed upon ionization in vapor or water to cause the release of gases in position of use. The use of water vapor is best because the gases then do not have a chance. of being washed from the fibrous structure or to dissolve in the aqueous medium. The use of such acidic substances here and in other systems described should be restricted to treatments wherein the glass fibers are resistant to the acidic media or exposed thereto for only short duration.

In carbonate systems and the like employing acidic medium for the liberation of gases, additional resinous and many substances may be used as part of the binding agent, such as carbowax, polyethylene,polystyrene, polyvinylidene chloride, gelatin and the like. Materials such as these are unaffected by-acids but will allow the acids to soak through and react with the carbonates to liberate carbon dioxide within the binder system. The calcium oxide or other by -product which remains will serve merely as additional bulking agent.

When acid forming saltsare used in the binder agent or mixed salts which form acid upon ionization, it is possible merely to soak the fibrous structure in aqueous medium and allow the moisture to permeate into the fabric and cause the-release of gases by reaction'of the chemical compound.

Example 13 About 3 parts by weight calcium carbonate is milled into a binder formed of 10 parts by weight gelatin and 20 parts by weight carbowax. The binder is formed into a dispersion which may be incorporated into a slurry of pulp binder fibers or used alone to form the binding agent in the formation of a glass fiber structure. After the fibrous structure has dried, the fibrous structure may be treated with an acid solution which permeates into the binder and evolves carbon dioxide as small microscopic bubbles that remains entrapped within the hinder or fibrous structure.

A dispersion of the above is incorporated with a slurry of newsprint and precipitated with glass fibers mixed therein upon a forarninous belt. The wet fibrous structure is exposed to vapors of hydrochloric acid which cause the release of carbon dioxide gas in the product. The fibers so treated may be dyed or a pigment may be employed as an ingredient with the binder. Instead of kaolin, other additives may be used such as talc, silica, bentonite, zinc oxide, titanium oxide, decalite, powder glass and the like.

(3) Gases absorbed or otherwise present in the diluent, such as dissolved gascsin the slurry may be freed to form microscopic bubbles in the formed fibrous structure by heat treatment or by ultrasonic vibration. For this purpose it is possible to dissolve gases in theslurry which are highly soluble therein undernormal conditions or under positive pressure. Representative are ammonia, carbon dioxide, sulphur dioxide and the like. Usually temperatures in excess of 400 F. are best for releasing the gases in situ on the glass fiber surfaces where they may be entrapped with the binder composition. Release of gases in position of use may be further aided by subjecting the fibrous structure to reduced atmosphere.

(4) Gases may be caused to be adsorbed in large quantity on the surface of certain substances incorporated or forming a part of the bonding agent and these may be released in position of use by thermal treatment or by supersonic or ultrasonic vibration.

Ingredients capable of having gases dissolved thereon for release in the fibrous structure and which may themselves find desirable use in the structure include the loading agents in colloidal form represented by talc, silica, kaolin, bentom'te, titanium oxide, diatomaceous earth, wood fiour, powdered glass fiber and the like. These may be dispersed in the slurry of pulp fibers as earlier described. The liberated gases cause foaming of the resinous materials if employed, or else remain entrapped with the pulp fibers which serve as a bonding agent and cause the desired separation and spacing between the glass fibers at their intersections.

The binding agent may be formulated with a plasticizer or diluent which is not removed under normal treatment for drying the formed fibrous mass. However, upon subsequent treatment under more rigid conditions the lower boiling diluent or plasticizer may be released as vapor which forms as small bubbles which may be entrapped in the fibrous structure to cause the desired separation of fibers. Treatment to release the vapor may be in the form of high temperature insufiicient to decompose the binder but sufficient to volatilize the lower boiling plasticizer or solvent. Ultrasonic or supersonic vibration may also be used.

(6) Highly porous additives may be incorporated with the fibrous materials in forming the fibrous structure. Such additives include expanded perlite and exfoliated vermiculite. These mineral substances beneficially affect the fibrous structure but their incorporation in uniform distribution has been difiicult to achieve because of their low specific gravity, as will hereinafter be pointed out.

When temperature is employed for the purpose of liberating the gaseous medium to form bubbles, it is possible in structures of the type described to employ temperatures in excess of 1000 F. whereby gaseous liberation can be achieved at a desirable rapid rate. Temperatures such as these cannot be used in the organic systems heretofore employed.

In a new and novel system microscopic air bubbles may be incorporated directly into the fibrous structure in forming. As illustrated in FIGURE 2, water 70 is recirculated through tank 71 from an inlet pipe 72 at the front end to an outlet 73 at the opposite end. Glass wool fibers 74 and the binder pulp 75 is fed into the tank at the front end. The fibers naturally fall en masse towards the bottom of the tank. Just before the fibers reach the bottom, very fine air bubbles 76 are introduced from the underside by injection of air under high pressure through a microporous screen 77 located in the bottom wall 78 of the tank.

Instead of introducing the microscopic air bubbles through a screen, it may be more practical to embody a flotation system wherein microscopic air bubbles are introduced into the lower portion of the tank 81 intermediate its ends by formation and actuation of a paddle wheel located alongside the tank. Air is pumped through an annular opening 82 about the driving shaft 83 and broken down into a froth or bubbles by reaction with the turning blades 84 of the wheel. The shaft is driven by a pulley 85 connected to a driving motor 86 by a belt 87. The froth or microscopic bubbles seem to attach themselves onto the fibers 88 introduced into the tank as 'described in connection with FIGURE 2 whereby the pulp and glass fibers rise as a formed fibrous structure with the fibers in the desired arrangement where they are continuously removed as a formed web for further processmg.

The'concepts of mineral flotation are particularly 'well adapted in the practice of this phase of the invention, especially when it is desired to manuufacture a fibrous structure of low density. Depending upon the particular composition of the dispersed fibers, fret-hers, promoters, depressants, activators, sulphidizers and regulators may be employed as in flotation systems. The sulphidizer, such as phosphorous pentasulphide, sodium sulfide and the xanthates, function to provide a sulphide coating on the oxide surfaces of the glass fibers to render them more receptive to attachment of bubbles. The collectors and promoters, such as alkaline metal salts, function to collect into aggregates the fibers and attach the fibers to the bubble. The frothing agent, such as pine oil, higher alcohols, coal tar creosotes and the like, lowers the surface tension of the liquid and supplies the bubble essential for attachment and rise of the fibers. The regulator, which is an alkaline substance such as lime, or an acid such as sulphuric acid, operates to adjust the hydrogen ion concentration to that which is most suitable for the process.

As the bubbles 76 rise up into the fibrous mixture, such agitation is maintained by mixers 78 and the rising bubbles as will cause the bubbles to become entrapped in the fibrous mass. The buoyancy added by the bubbles will cause the fibrous mass to rise to the surface where it can be picked up by a conveyor belt 79 which lifts the fibers as a felted layer out of the tank and over a suction box or drying equipment or both, as previously described.

Without squeezing, fibrous products having densities lower than five pounds per cubic foot may be produced. It appears that the pulp fibers of asbestos or cellulose, when used, orient themselves about the surfaces of the bubbles with the result that a structure having greately improved characteristics is secured.

More important is a system which makes possible the incorporation of expanded inorganic minerals of the type perlite and vermiculite as loading agents in fibrous structures of the type described. Although it is possible to mix expanded perlite in glass fiber mixtures having little water, the uniform incorporation of such expanded minerals with glass fibers has been diflicult to achieve in a highly diluted water system because such minerals naturally flow to the surface whereas the fibrous materials generally separate and flow towards the bottom of the aqueous system. By the use of the foaming agents such as the injection of air in the manner described through the bottom wall of the tank, the fibrous mass may be caused to rise to the surface as rapidly as the expanded perlite or vermiculite. The fibrous materials may become uniformly mixed with such expanded materials at the surface so that they may be led off together as a continuous fibrous layer from the surface of the tank for subsequent treatment as by compression or drying to form a fibrous layer.

Applicant has found that the concepts of ultrasonic or supersonic vibration may be advantageously employed in the various concepts of this invention. For example, the possible variations of vibrations from a high rate to a relatively low rate in the supersonic or ultrasonic range may have the effect in one instance of defloccuating the pulp fibers in the manner achieved by the use of metal chlorides and tannic acids with asbestos or by the use of metal chlorides and pulp fibers. At the point where the fibers are about to settle upon the separating screen, the supersonic effect may be changed to cause deflocculation of the fibrous materials so that they settle out together in the desired arrangement with the glass fibers in the formation of desired fibrous structures.

The supersonic or ultrasonic vibration principles may also be employed in the techniques for liberating gas bubbles during fabric formation so that the bubbles will remain and separate the fibers one from another. Bubble separation may be achieved by supersonic vibration of the aqueous medium in which gases have been absorbed in high concentration or from the surfaces of the particles in which such gases have been absorbed. Supersonic vibration may also be employed in the carbonate system either to impart acidic environment to the fiber forming com position whereby carbon dioxide is liberated from the corresponding carbonate or else supersonic vibration may be employed to provide high temperature which causes carbon dioxide. and like. gases to be released from chem cal compositions in which they form a part. The possibility of causing heat to be generated by supersonic vibration might also be adapted for the formation of vapors by vaporization of solvent or aqueous medium associated with the mat of the glass fiber structure.

It has been found that under certain circumstances, glass fibers may be made to function in a manner somewhat similar to the asbestos or cellulose pulp fibers for bonding other longer glass fibers into a fibrous structure.

Glass fibers ground so fine as to appear as a powder to the naked eye have been found to still be fibrous in nature under the electron microscope and when in such finely divided form, they have some of the clinging effect characteristic of the pulp fibers-heretofore described. Such fine-glass fibers appear to have a dimension of about 1-10 ten-thousandths of an inch or less. Thus glass fibers pulped and ground to finely divided form may be used in the techniques described to bond glass fibers of longer length into fibrous structures. This is considered to be an important advance in the technology of glass fibers and fibrous structures. It makes available for the first time a complete felted glass fiber fabric which does not require other substances to impart strength and integrity thereto.

It will be manifest from the description that I have provided a number of concepts which may be used in the manufacture of new and improved fibrous structure, capable of many new uses and adapted to function better in many applications already developed for glass fiber products. The product of this invention is not subject to the limitations imposed by the presence of large quantities of organic material but may be used successfully in applications wherein the product will be subjected to temperatures in excess-of 500 F. or even where it might be exposed to direct flame. From a practical standpoint, importance resides also in the economy of manufacture of the types described because fiber which does not separate to form the structure is not lost but may be returned with the water to form fibrous suspensions.

The product of this invention is a highly porous prodnot which has exceptional flexure strength and flexibility and may be varied from a board of high density and strength to a porous insulation product.

It will be understood that numerous changes may be made in materials, composition and apparatus without departing from the spirit of the invention, especially as defined in the following claims.

a I claim: 7

1. In the method of manufacturing a fibrous structure of glass fibers and pulp fibers wherein the glass fibers constitute the principal component of the structure to determine the cross-section thereof and the pulp fibers are disposed between the glass fibers to interbond the 7 glass fibers one to the otherin the structure, the'steps of advancing a preformed batt of glass fibers continuously througha slurry, the fibrous component of which consists essentially of pulp fibers, circulating the slurry in ,a direction crosswise of the direction of travel of the glass fiber'batt for passage throughthe batt from one side to the other whereby pulp fibers separate'from the slurry during passage therethrough to interbond the glass fibers, conveying the structure of glass and pulp fibers out of the slurry, and then drying the fibrous structure.

2. The method of forming a fibrous structure comprismg the steps of providing a bonded layer of glass fibers, impregnating the layer of glass fibers with a slurry of cellulosic pulp fibers containing from 1 to 5 percent by weight of a colloidal silica whereby to achieve substantially complete penetration oftheglass fiber layer and then drying the fibrous layer with the pulp fibers therein as. a binder. V

3. In. the. me h d of m l ufacturing a fibrous struc- .ture,.the steps of providing a layer of felted glass fibers, impregnating the fibrous layer with a slurry of asbestos pulp, first adjusting the pH of the slurry to just below neutral to increase the penetrating power thereof, adjusting the pH to between 7.0 and 8.5 to coagulate the asbestos fiber when in contact with the glass fiber layer, and then drying the fibrous layer to form the fibrous structure. a

4. The method of manufacturing a fibrous structure,

comprising the steps of forming a dispersion of glass fibers, asbestos fibers and an acidic compound to flocculate the asbestos pulp fibers and increase the fluidity of the slurry, deflocculating the asbestos pulp fibers after the dispersion is passed on to a collecting wall which separates out the fibers into a felted layer while the water drains therethrough, and drying the felted layer to form the fibrous structure. 5. The method of manufacturing a fibrous structure comprising steps of forming a layer of glass fibers with a coagulating agent dried onto the surface thereof, treating the dried glass fiber layer with a'slurry of asbestos pulp fibers adjusted to acid conditions to slightly below pH 7, to increase the penetrating power of asbestos pulp fibers in the slurry, the pulp fibers becoming coagulated upon contact with the deflocculating agent on the glass fiber surfaces whereby the pulp fibers become separated and attach themselves to the glass fiber surfaces.

6. The method of manufacturing a fibrous structure as claimed in claim 5, which includes the additional step of treating the fibrous structure with a resinous material to increase the strength thereof. 7 V 7. In the method of manufacturing a fibrous structure of low density, the steps of incorporating a carbonate compound in particle form with glass fibers bonded into a self-sufficient mass with a binder consisting essentially of pulp fibers uniformly distributed throughout the mass and selected from the group consisting of asbestos pulp and cellulose pulp, and acidifying the mass while wet with aqueous medium to cause release of carbon dioxide from the carbonate particles for entrapment iuthe wet fibrous mass to increase the porosity thereof.

8. In the method of manufacturing a fibrous structure of low density, the steps of incorporating a carbonate compound in particle form and a compound which forms an acidic reaction upon ionization in water with glass fibers bonded into a self-sufficient mass with a binder consisting essentially of pulp fibers uniformly distributed throughout the mass and selected from the group consisting of asbestos pulp and cellulose pulp, wetting the bonded mass with aqueous medium to cause acid formation and release of carbon dioxide from the carbonate particles for entrapment in the wet fibrous massto increase the porosity thereof. a a a 9. In the method of manufacturing a fibrousstructure of low density, the steps of incorporating a compound in particle form, which is capable of, releasing a gas upon reaction, with glass fibers bonded into a self-sufiicient mass with a binder consisting essentially of pulp fibers uniformly distributed throughout the mass andselected from the group consisting of asbestos pulp and cellulose pulp, wetting the mass of glass fibers with an aqueous medium, and treating the mass While wet to cause a release of gas from the particles which gas becomes entrapped in the wet fibrous mass to increase the porosity thereof. a v

10. In the method of manufacturing a fibrous structure of low density as claimed in claim 9 in which the gas releasing compound comprises a carbonate compound in particle form capable of releasing carbon dioxide. 7

1 1. In the method of manufacturing a fibrous structure of low density, the steps of incorporating a carbonate compound in particle form with glass fibers bonded into a self-suflicient mass with a binder consisting essentially of pulp fibers, subjecting the fibrous structure with the carbonate particles dispersed therein to supersonic vibration causing release of carbon dioxide from the carbonate particles.

12. In the method of manufacturing a fibrous structure comprising the steps of depositing glass fibers dispersed with a slurry of pulp fibers onto a collecting wall to separate out the fibers as a felted layer while the diluent passes therethrough, first dissolving gases in the slurry and then treating the wet layer of glass and pulp fibers to release dissolved gases and which remain entrapped in the fibrous structure.

13. In the method of manufacturing a fibrous structure, the steps of forming, by separation, a web from a slurry of components comprising glass fibers as a fibrous component, pulp fibers as a binding component and finely divided inorganic particulate substance having a gas adsorbed on the surface thereof whereby the particulate substance remains dispersed throughout the fibers within the formed web and heat treating the fibrous structure to cause the release of adsorbed gases from the particulate substance to form microscopic bubbles, some of which remain entrapped in the fibrous structure.

14. The method of manufacturing a porous structure of glass fibers comprising introducing a slurry of glass and pulp fibers into one end of a container wherein the fibers settle to form a web, passing microscopic air bubbles into the bottom of the bath in the region where the fibrous Web approaches the bottom, permitting the bubbles to rise into the fibrous web and become entrapped therein causing the fibrous Web to rise to the surface of the bath, and then conveying the formed fibrous web with the bubbles therein to further processing units.

15. In the method of manufacturing a fibrous structure, the steps of forming, by separation, a web from a slurry of components comprising glass fibers as a fibrous component, pulp fibers as a binder component and fine- 1y divided inorganic particulate substance having a gas adsorbed on the surface thereof whereby the particulate substance remains dispersed with the glass fibers throughout the formed web and vibrating the fibrous structure to cause the release of adsorbed gases from the particulate substance to form microscopic bubbles, some of which remain entrapped in the fibrous structure.

References Cited in the file of this patent UNITED STATES PATENTS 688,420 Kelly Dec. 10, 1901 1,740,280 Bryant Dec. 17, 1929 1,878,047 Wenger Sept. 20, 1932 2,008,141 McClellan July 16, 1935 2,083,132 Williams et a1. June 8, 1937 2,152,901 Manning Apr. 4, 1939 2,156,308 Scbuh May 3, 1939 2,162,386 Neuhof June 13, 1939 2,184,316 Plummer Dec. 26, 1939 2,189,840 Simison et al. Feb. 13, 1940 2,200,155 Camp et al. May 7, 1940 2,286,968 Lundt June 16, 1942 2,300,137 Salisbury Oct. 27, 1942 2,312,776 Rankin Mar. 2, 1943 2,316,998 Smith Apr. 20, 1943 2,444,347 Greger June 29, 1948 2,504,744 Sproull et al. Apr. 18, 1950 2,626,213 Novak Jan. 20, 1953 2,653,090 Crandall Sept. 22, 1953 2,747,994 Hoopes May 29, 1956 2,772,157 Cil-ley et a1 Nov. 27, 1956 FOREIGN PATENTS 497,059 Great Britain Dec. 8, 1938 

7. IN THE METHOD OF MANUFACTURING A FIBROUS STRUCTURE OF LOW DENSITY, THE STEPS OF INCORPORATING A CARBONATE COMPOUND IN PARTICLE FORM WITH GLASS FIBERS BONDED INTO A SELF-SUFFICIENT MASS WITH A BINDER CONSISTING ESSENTIALLY OF PULP FIBERS UNIFORMLY DISTRIBUTED THROUGHOUT THE MASS AND SELECTED FROM THE GROUP CONSISTING OF ASBESTOS PULP AND CELLULOSE PULP, AND ACIDIFYING THE MASS WHILE WET 